1. Field of the Invention
The present invention relates to a bottom-to-surface method and system for an underwater pipe installed at great depth.
The technical sector of the invention is the field of manufacturing and installing rising production columns for underwater extraction of oil, gas, or other soluble or fusible materials or a suspension of minerals from an underwater well head for the purpose of developing production fields installed at sea off-shore. The main application of the invention lies in the field of oil production.
2. Description of the Related Art
The present invention relates to the known field of links of the type comprising a vertical tower anchored to the sea bed and having a float situated at the top of the tower, which float is connected to a floating support installed on the surface by means of a pipe whose own weight causes it to take up the shape of a catenary.
In the present description the production fields are considered as being oil fields. Once the underwater depth of such fields becomes large, they are generally worked from floating supports. The well heads are often distributed over the entire field and production pipes and also water injection lines and control command cables are placed on the sea bed going towards a fixed location having a floating support positioned vertically above it on the surface.
In general, the floating support has anchor means so as to enable it to remain in position in spite of the effects of current, wind, and swell. It also generally includes means for storing and processing oil and means for off-loading it to off-loading tankers, which arrive at regular intervals to take away the production. These floating supports are known as xe2x80x9cfloating production storage and off-loadingxe2x80x9d (FPSO) supports and the initials xe2x80x9cFPSOxe2x80x9d are used throughout the description below to designate such a support.
Such FPSOs are either anchored by a series of anchor lines running from each of the corners of the floating support, in which case the FPSO maintains a substantially constant heading regardless of surrounding conditions, or else an FPSO has a turret secured to its structure and anchored by a series of anchor lines. Under such circumstances, the FPSO is free to revolve relative to the turret, and it is the turret that maintains a constant heading; under such circumstances the FPSO takes up a heading that corresponds to the resultant of the forces due to wind, current, and swell on the hull of the vessel. In the following description, the bottom-to-surface links are described for the most part as being connected to the side of an FPSO that is anchored and that therefore has a substantially constant heading (as shown in FIG. 2), whereas if the FPSO has a turret, then they should be connected to the turret itself (as shown in FIG. 6).
The bottom-to-surface link pipe is known as a xe2x80x9criserxe2x80x9d, which term is used in the present description, and it can be implemented in the form of a pipe rising continuously from pipes placed on the sea bed and going directly to the FPSO, thereby giving rise to a catenary configuration whose angle relative to the vertical at the FPSO is generally in the range 3xc2x0 to 15xc2x0 (a catenary riser).
When the water depth is less than several hundred meters such links must necessarily be made using pipes that are flexible, however once the depth reaches or exceeds 800 m to 1000 m, flexible pipes can be replaced by pipes that are strong and rigid, being constituted by tubular elements that are welded or screwed together and made of rigid material, such as composite material or thick steel. Such rigid risers of thick strong material and taking up a catenary configuration are commonly referred to as xe2x80x9csteel catenary risersxe2x80x9d (SCRs) and the initials SCR are used in the present description regardless of whether the riser in question is made of steel or of some other material such as a composite.
A flexible pipe and an SCR type rigid riser when subjected to the forces of gravity only, and when they are of the same height, present the same angle relative to the vertical where they connect to the FPSO, and have the same curvature over their entire suspended length. Mathematically, this curve is accurately defined and is known as a xe2x80x9ccatenaryxe2x80x9d. However, SCRs are much simpler than flexible pipes technically speaking and they are much less expensive. Flexible pipes are structures which are complex and expensive and which are made from multiple spiral-wound sheaths and composite materials.
The depth of certain oil fields is greater than 1500 m and can be as great as 2000 m to 3000 m. The tension induced at the FPSO by each SCR can be as great as 250 metric tonnes to 300 tonnes and the large number of risers needed to develop certain fields leads to reinforcing the structure of said FPSOs considerably, and can give rise to unbalance if starboard and port loading is not the same. In addition, during circular movements of the FPSO about its mean position, the catenary formed by an SCR changes and the point of contact on the sea bed moves forwards and backwards and also from left to right at the same rate as the FPSO moves, putting down or picking up a portion of the pipe. These movements are repeated over long periods of time and they dig a furrow in poorly consolidated beds of the kind commonly encountered at great depth, thereby modifying the curvature of the catenary and leading, if the phenomenon amplifies, to risks of the pipes being damaged, i.e. the underwater pipes can be damaged and/or the SCRs can be damaged.
Because of the multiplicity of lines that exist in installations of this type, it is preferred to use a solution of the tower type in which the pipes and cables converge on the foot of a tower and rise up the tower, either all the way to the surface, or else to a depth that is close to the surface, with flexible pipes then extending from that depth to provide links between the top of the tower and the FPSO. The tower is then provided with buoyancy means so as to keep it in a vertical position and the risers are connected at the foot of the tower to the underwater pipes via flexible coupling sleeves which accommodate the angular movements of the tower. The resulting assembly is commonly referred to as a xe2x80x9chybrid riser towerxe2x80x9d since it makes use of. two technologies: firstly a vertical portion, the tower, in which the riser is constituted by rigid vertical pipes; and secondly a top portion of the riser which is constituted by flexible pipes in a catenary configuration connecting the top of the tower to the FPSO.
French patent FR 2 507 672, which corresponds to U.S. Pat. No. 4,462,717, discloses such a hybrid tower comprising a surface float connected to the FPSO via flexible pipes and carrying suspended guides through which there pass solely the top portions of the vertical fluid transfer pipes. The hybrid tower is anchored to the sea bed by a cable under tension that gives the assembly a certain amount of flexibility in vertical movement, the bottom portions of the pipes being free and forming bends at the sea bed, against which they bear.
The advantage of such a hybrid tower lies in the freedom allowed to the FPSO to move away from its normal position while giving rise to a minimum amount of stress in the tower and in those portions of the pipes that are in the form of suspended catenaries, whether at the sea bed or at the surface. The FPSO is generally anchored by means of a multitude of lines connected to a system of anchors resting on the sea bed. Such an anchor system gives rise to return forces that maintain the FPSO in a neutral position. The bottom-to-surface links give rise to additional vertical and horizontal forces which have the effect of offsetting the axis of the FPSO relative to said neutral position. In the absence of current, wind, or swell, and when the tide is at its mean level, the position of the FPSO corresponds to a xe2x80x9creference positionxe2x80x9d P0. Under the combined effects of environmental conditions, both on the hull of the FPSO and also on the various elements constituting the risers, the FPSO will move away from said reference position in proportion to the resultant of all the forces applied to the system.
Thus, for forces on the hull of the FPSO tending to move it away from the axis of the tower, the following effects are observed: firstly the catenary is stretched and its angle relative to the vertical at its point of attachment to the FPSO increases, thereby increasing the vertical and horizontal forces on the FPSO; and secondly the angle of inclination of the tower due to said horizontal force also increases.
In order to minimize the consequences of FPSO excursions, it is general practice to increase the stiffness of the anchor system and to provide flexibility in the bottom-to-surface links. For this purpose, the tower configuration associated with the catenary has a large capacity to absorb FPSO excursions, while minimizing movements of the tower and deformation of the catenaries.
To damp the movements of the FPSO, it is desirable to increase the curvature of the pipe that connects it to the top of the tower. Flexible pipes are believed to be better adapted to making links between an FPSO and the top of a tower. In prior embodiments of xe2x80x9chybrid towersxe2x80x9d as described in FR 2 507 672 or in other types of structure such as those described in U.S. Pat. No. 4,391,332 and EP 0 802 302, use is made of plunging flexible pipes, i.e. pipes that go down to a depth well below the float before subsequently rising again. This is possible since a flexible pipe is capable of withstanding fatigue even when its curvature presents a radius of curvature of only a few meters.
However, the internal structure of flexible pipes is very complex and their cost very high, that is why prior embodiments of hybrid towers have sought to raise the tower as close as possible to the surface while nevertheless avoiding the turbulent zones at the surface, i.e. the top of the tower is to be found at a depth that is generally no more than 200 m, and preferably about 50 m. This makes it possible use short lengths of flexible pipe that are therefore less costly, and above all this makes it possible to ensure that the connections between the flexible pipes and the top of the tower are made more accessible to divers.
All of the elements of such hybrid towers or of such catenary risers must be dimensioned so as to be capable of withstanding swell, current, and movements of the surface vessel under extreme sea conditions, which leads to immersed structures of considerable size capable of withstanding high levels of stress and of withstanding fatigue phenomenon throughout their lifetime, which commonly reaches or exceeds 20 years.
The problem posed is thus to be able to make and install such bottom-to-surface links for underwater pipes at great depth, e.g. deeper than 1000 meters, and of the type comprising a vertical tower anchored to the sea bed and whose top float is connected to a floating support installed on the surface via a pipe in the form of a catenary, while nevertheless limiting forces on the floats and the pipes connecting it to the floating support, the entire system being capable of withstanding the stresses and fatigue while nevertheless accommodating large displacements of the surface support without requiring structures that are large and too expensive, and which should be capable of being put into place easily and reversibly so that they can easily be maintained and replaced.
A solution to the problem posed is a bottom-to-surface link system for an underwater pipe installed at great depth, the system comprising firstly a vertical tower constituted by at least one float associated with an anchor system and carrying at least one vertical riser connecting the float to the sea bed and capable of being connected to underwater pipes resting on the sea bed, and secondly at least one link pipe extending from said float to a surface support, such that, according to the present invention, said link pipe is a riser whose wall is a strong rigid tube, in particular made of steel or of composite material.
For a rigid pipe, the minimum acceptable radius of curvature is 10 to 100 times greater than that of a flexible pipe. To limit fatigue, it is accepted that the radius of curvature of a rigid pipe made of steel should generally be greater than about 100 m. To provide flexibility and achieve identical capacity to absorb the movements of the floating support and the movements of the tower, the fact that the catenary is less curved when using a rigid pipe is compensated by increasing the distance between the floating support and the float at the top of the tower, and thus by increasing the length of the rigid pipe. However, the apparent weight in water of a rigid pipe is greater than that of a flexible pipe, so the load at the float and the forces on the float at the top of the tower are therefore increased. This could lead to the float being overdimensioned, thereby leading to high levels of cost. That is why it is preferable, in accordance with the present invention, to install the top float of the tower at a greater distance from the surface of the water, and in particular at a depth that is below the last thermocline (where xe2x80x9cthermoclinexe2x80x9d is defined below), and preferably not less than 100 m beneath the last thermocline. In particular, the top float of the tower is installed at least 300 m below the surface of the water, and preferably at least 500 m below the surface of the water, and more preferably at a depth that is greater than half the depth of the water in which the tower is anchored.
By lowering the top float of the tower in this way, the following advantages are obtained simultaneously:
the length of the rigid pipe providing the link between the FPSO and the top of the tower is increased, thereby providing greater damping of the movements of the tower and of the FPSO;
the minimum acceptable radii of curvature for a rigid pipe in a catenary are nevertheless complied with, regardless of how much the system as a whole moves; and
costs are minimized since for a shorter tower the underwater structure is less massive and therefore less expensive and the float required for putting it under tension is smaller and therefore less expensive, and this is true in spite of the increase in the apparent weight in water of the pipe associated with its increased length. This is because the catenary does not rise or rises very little towards the float, so the weight of the rigid pipe constituting the catenary is essentially supported directly by the FPSO.
Nevertheless, maintaining a tower of a certain height, in particular not less than 50 m and preferably not less than 100 m is advantageous since by being able to move the tower contributes to damping the system under the effect of movements of the FPSO.
In a preferred embodiment, the anchor system has at least one vertical tendon, a bottom foot unit to which the bottom end of the tendon is fixed, and at least one guide through which the bottom end of said vertical riser passes. More particularly, the guide can be on the foot unit. Advantageously, said tendon also has guide means distributed along its entire length, through which at least said vertical riser passes.
Said foot unit can merely be placed on the sea bed and stay in place under its own weight, or else it can be anchored by means of piles or any other device suitable for keeping it in place; the float is connected to said foot unit via a flexible connection situated at the foot and via an axial link constituted either by a cable or by a metal bar or indeed by a pipe. The axial link is referred to in the present description as a xe2x80x9ctendonxe2x80x9d.
In a preferred embodiment, the top end of the vertical riser is suspended through at least one guide secured to the float, placed within the float, or at the periphery thereof. The top end of the vertical riser is connected via the top of the float to the bend at the end of the link pipe, and the bottom end of the vertical riser is suitable for being connected to the end of a connection sleeve that is likewise bent, and that is movable between a high position and a low position relative to said foot unit. The sleeve is suspended from the foot unit and is associated with return means urging it towards its high position in the absence of the riser, the return means possibly being constituted by a counterweight. By having a connection sleeve that is movable in this way, variations in the length of the riser under the effects of temperature and pressure can be accommodated.
At the top of the vertical riser, an abutment device secured to the riser bears against the support guide installed at the top of the float and thus supports the entire riser: the riser is then suspended with its apparent weight in water being supported by part of the buoyancy of the float.
In a particular embodiment, each of said guide means distributed along the entire length of the tendon and through which said vertical riser passes comprises a cylindrical cavity, preferably surmounted by a conical funnel, with the inside diameter of the cylindrical cavity being greater than the diameter of the vertical riser, and each of said guide means has a flexible membrane secured to the inside wall of its cylindrical cavity, thereby creating a leakproof bag between said membrane and said inside wall, which bag can be filled with a fluid, preferably of very high viscosity, so as to bear against the riser.
Friction shoes are preferably associated with said membrane so as to bear against the riser when said bag is filled with fluid. The shoes thus enable the vertical riser to slide when its length varies under the effects of temperature and pressure.
The objects of the present invention are also obtained by a link method making use, as explained above, firstly of a vertical tower constituted by at least one float associated with an anchor system and carrying at least one vertical riser suitable for going down to the sea bed, and secondly at least one link pipe from said float to a surface support, whereby, in the present invention, said float is immersed at a depth situated below the last thermocline (where xe2x80x9cthermoclinexe2x80x9d is defined and explained below), and said float is connected to the surface support via at least one strong rigid riser constituting one of said link pipes.
In a preferred implementation of the link method of the invention:
a foot unit is put into place on the sea bed and secured to said bed; the bottom end of a tendon is secured thereto with the opposite, top end of the tendon being secured to said float, the assembly constituting said anchor system of the vertical tower;
said vertical riser is progressively lowered e.g. from a floating support located vertically above said float, through one of the guide assemblies thereof until its top end comes to bear against said float, its bottom end then being connected to the top end of a coupling sleeve preinstalled on said foot unit.
As it moves down, the vertical riser preferably passes in succession through a series of guides secured to the axial link, referred to as a xe2x80x9ctendonxe2x80x9d, thereby ensuring that it is held in a position that is substantially parallel to said tendon and to the other vertical risers, whether already installed in adjacent guides, or to be installed at a later date.
In a particular implementation, said float is installed so as to be immersed at a depth that is greater than half the depth of the water in which the tower of the invention is anchored, thus making it possible to assemble the entire vertical riser prior to installing it and to transport it to a position vertically above the guide corresponding to the float so as to be lowered therethrough.
The result is a novel bottom-to-surface link method for an underwater pipe installed at great depth and satisfying the problem posed.
Studies of sea currents in various seas over the world show that various layers exist starting from the surface and going down to the sea bed. Thus, at depths in excess of 500 m to 1000 m, in an Atlantic Ocean type configuration, the following is observed, as shown in FIG. 1:
a surface layer 181 that can go down to about 50 m below the surface 19 and in which currents are local and mainly due to wind and tide phenomena. In this zone, currents are large and substantially uniform over the depth of the layer. They can have speeds of as much as 2.5 meters per second (m/s) off West Africa;
a transition zone 291 known as a xe2x80x9cthermoclinexe2x80x9d, can be of various thickness but which is always of small thickness (3 m to 10 m). In this transition zone 291, the current falls off quickly to match the speed of the intermediate layer;
an intermediate layer 182 in which currents lie in the range 0.5 m/s to 1 m/s. This intermediate layer extends from about xe2x88x9255 m to about xe2x88x92150 m and the currents are mainly thermal currents due to climatic phenomena;
a second transition zone 292 or xe2x80x9cthermoclinexe2x80x9d which is likewise of various thicknesses but always of small thickness (≈10 m). In this transition zone, current falls off quickly to match the current in the bottom layer; and
a bottom layer 183 in which currents are small, generally not exceeding 0.5 m/s. These currents are due to intercontinental movements of water. This layer begins at about xe2x88x92150 m to about xe2x88x92170 m and it continues all the way down to the sea bed 12, i.e. down to depths that can be as great at 1000 m to 3000 m, depending on location.
In certain seas, three ethermoclines 29 can be observed in the upper portion, but as a general rule the bottom layer 183 begins at around xe2x88x92170 m to xe2x88x92200 m.
Thus, since the tower and its float in accordance with the invention and as described below are located below the bottom thermocline 292 they are to be found in a layer of water 183 that gives rise to the smallest stresses due to current. In addition, the float is protected from the effects of swell, which effects fall off quickly with depth, and it is common practice to ignore them once the depth exceeds 120 m to 150 m. The forces to which the tower is subjected are thus considerably reduced and substantially uniform over its entire height since they are due to intercontinental deep currents.
The system of the invention constituted by a tower associated with an SCR thus provides much better behavior in response to environmental conditions, both ordinary conditions and extreme conditions such as once-yearly conditions, 10-year conditions, and 100-year conditions. The forces and the stresses are very considerably reduced and the fatigue behavior of the various critical components is considerable increased, thereby making it possible to deliver better service throughout the lifetime of the field.
The float is thus at considerable depth, and it can be connected to the FPSO via at least one SCR instead of being connected via a flexible link as is the present practice. SCR links are simple and in addition, the internal structure of the SCRs, the vertical risers, and the pipes resting on the sea bed can then be identical, thereby simplifying the passage of cleaning scrapers. It is essential for such cleaning scrapers to be passed frequently when solid deposits such as paraffin or hydrates occur, and it must be possible to take action in repeated and highly energetic manner without damaging the inside surfaces of the risers and the pipes.
In general, the float is installed at about half the total water depth, but it could be installed higher or lower in order to take advantage of certain situations as described below. In any event, the float is never situated close to the last thermocline as described above but always at some greater depth, e.g. 100 m below it, so as to ensure that it runs no risk of being subjected to the disturbances generated by the thermocline, nor to the currents that exist in the top layer in the event of planet-wide disturbances in sea currents significantly altering ocean movements.
The SCR is connected to the vertical riser at the top of the float via a flexible joint which enables the angle between the axis of the tower and the axis of the catenary at said flexible joint to vary widely without imparting significant stresses to the SCR or to the top of the float. The flexible joint can either be a ball-and-socket type joint with sealing gaskets, or else it can be a layered ball made up of a sandwich of elastomer sheets and metal sheets bonded together and capable of absorbing large amounts of angular movement by deforming the elastomers while nevertheless maintaining complete leakproofing because of the absence of any rubbing surfaces, or indeed it could be a short length of flexible pipe capable of providing the same service.
The system of the invention is advantageously fitted with an automatic connector situated at the flexible joint, either between the tower and the flexible joint or between the flexible joint and the FPSO. Thus, such an SCR can be installed in a manner that is entirely automatic without requiring the use of divers. The installation sequence then consists in installing the tower, then in transporting the future SCR in a vertical position, and fixing it to the side of the FPSO in its final position. A cable connected to the bottom end of the future SCR is then manipulated by a remotely operated vehicle (ROV) so as to be brought to the top of the tower and so as to be connected to hauling means secured to the float and controlled e.g. by the ROV which then supplies the necessary power while also monitoring operations by means of video cameras whose signals are taken to the surface for use by operators located on a floating service vessel. The cable is then hauled in and the end of the SCR fitted with the male endpiece of an automatic connector (for example) is brought up to the female endpiece of the same automatic connector. At the end of the approach stage, the assembly is locked together and the hauling means are released so as to be capable of being used for installing the next line. The principle of automatic connectors is well known to the person skilled in the art of hydraulics and pneumatics, and is therefore not described in greater detail herein.
This method of installation presents the advantage of being entirely reversible, insofar as the automatic connector is designed to be capable of being disconnected. It is thus possible, in operation, to act on a single SCR for the purpose of disconnecting it and replacing it without disturbing the rest of production, and thus without any need to stop production on adjacent risers and SCRs.
Similarly, the tower and the vertical risers are advantageously installed using the following sequence:
the foot unit is put into place and secured to the sea bed;
a tendon fitted with guides and with the top float is installed;
the assembled vertical riser is transported in the vertical position so as to be vertically above its guide situated in the float;
the vertical riser is lowered progressively through its guides with the lowering operations being monitored from the surface;
at the end of being lowered, the head of the riser rests on the top of the float and includes a bend and also, for example, the flexible joint which has the female portion of the above-described automatic connector secured thereto; and
the bottom end of the vertical riser is also advantageously fitted with an automatic connector, preferably with the male portion thereof because it is smaller, and the assembly can be connected to the end of the underwater pipe connecting the foot of the tower to one of the well heads, said end being fitted with the female portion of said automatic connector.
Installing the vertical risers in this way presents the advantage of being entirely reversible, insofar as the automatic connector at the foot of the riser is likewise designed to be capable of being disconnected. It is thus possible in operation to act on a single riser so as to remove it and replace it without disturbing the rest of production, and thus without any need to stop production in adjacent risers and SCRs.
Insofar as the float is installed at a depth of more than half the total depth of the water, the fully assembled riser can be transported in the vertical position and lowered through the float. If the float is higher than half the depth of the water, the vessel used for installing the riser should be positioned vertically above the float and elements of the riser should be assembled to one another as the bottom end thereof is lowered through the float and the various guides installed along the tendon, with such assembly being implemented, for example, by welding, by adhesive, or indeed by mechanical assembly such a screwing, bolting flanges together, or crimping. In a preferred version of the system, a preassembled length of riser is transported in the vertical position from an assembly site that is remote from the tower, said length being shorter than the depth of water that remains between the surface and the top of the tower. In this way, the service vessel can take up position vertically above the float with a good length of riser pre-assembled and fitted at its bottom end with the male portion of the automatic connector, ready to be lowered towards and through the float and through the various guides installed along the tendon. As it moves down, the missing top portion of the riser is assembled as described above.
The above-described method of operation makes it possible to minimize the length of time the service vessel is present in the vicinity of the tower, thereby minimizing the risk of accident. Thus, in order to be able to take action at a later date and remove the riser in simple manner, it is preferable to use assembly methods that are suitable for rapid and non-destructive disassembly, such as screwing, thereby enabling the riser to be extracted from its supports, enabling it to be disassembled by unscrewing successive segments of the top portion, but only in sufficient numbers to release the bottom portion of the riser from the top of the float, after which the service vessel can change position together with the remainder of the riser suspended therefrom, heading for a location that is remote from sensitive installations prior to terminating maintenance operations.
In order to minimize the presence of the service vessel vertically above the tower, it is advantageous to install the float at a depth that is greater than half the total depth of the water, thus making it possible for the service vessel to install or extract an entire riser without needing to assemble or disassemble any of its components, thereby further reducing the risk of accident in the vicinity of the tower and of the sensitive installations.